Powder metallurgy is a process used to produce finished metal products similar to those manufactured in the steel industry. However, in the steel industry, various raw materials are mixed and melted in furnaces. The molten steel is poured into ingots and allowed to cool to form solid steel. The steel can then be worked and machined into products. In powder metallurgical processing, the starting material is a metal powder which is placed into a mold similar in shape to the desired finished product. This results in subsequent formation of a product that is near-net shape, to the desired finished product. The mold is typically rubber-like and is placed into a steel container so it will retain its shape when the powder is loaded into it. If a hole or cavity is desired in the finished product, it can be formed in the powder by using a solid shape of calculated size inside the mold. The shape (called a mandrel) is surrounded by the metal powder. After the required amount of powder is loaded into the mold, the mold is sealed by either mechanical means or liquid rubber, thus encapsulating the powder with rubber. Then, depending on the type of mandrel used, the mandrel is either extracted (removal type) or left in the mold (solid type). The apparatus is then isostatically pressed (pressure from all directions) using an oil or water medium. The part is now in the "green" or as-pressed state. The part can be handled and moved, but if it is dropped or jarred, it will chip or crack. The as-pressed part is placed in a furnace and heated to a temperature below its melting point to result in coalescence of the powder particles and formation of a solid article. This process is called sintering. The as-sintered part is the finished product. If necessary the as-sintered product can be worked or machined.
Therefore, the two main advantages of powder metallurgy are (1) a minimum weight of material is required to make the part due to forming to near-net-shape, and (2) there is a reduced energy consumption since the metal does not have to be melted.
The one disadvantage of powder metallurgy processing is the inability to achieve theoretical density of the metal without extraordinary means. To fully appreciate this problem, the shrinkage/density change from powder to as-sintered article must be understood. Metal powder is approximately 30% of theoretical density. The as pressed or green part is approximately 60% of theoretical density, and the as-sintered part usually has a density range of from about 92% to about 97% of the theoretical density. As the density increases, the part shrinks. This shrinkage can be calculated and is commonly referred to as the shrink factor.
The shrinkage factor becomes more complicated with tubes and crucibles. If a solid mandrel (one that remains in the mold during pressing) is used, the shrinkage from the green to sintered state must be used in determining the cavity size. If a removable mandrel (one that is extracted from the mold before pressing) is used, the powder to sintered shrink factor must be used. When a removable mandrel is used, another mold must initially encapsulate the mandrel. This is because the mandrel is extracted before pressing and the cavity left after extraction is filled with water during pressing. Now, as the part is shrinking, it, of course, contracts toward the center. However, if multiple cavities are to be formed, each cavity itself shrinks and all the cavities shrink toward the center of the part.
The ability to form more than one cavity in a powder metal isostatically pressed part as opposed to machining the multiple cavities in the part has two major advantages. They are that (1) less feed material is required, and (2) less machining time is required since less material is in the sintered blank.
The present invention provides a method for providing powder metallurgically produced isostatically pressed parts having a plurality of cavities.